It's an interesting idea.
I did a few calculations and to get even a modest 20.5" wide "marquee", you would need a 42" LCD (plasma, whathaveyou). If you used a standard 8" or so for the height, that leaves you with a vertical screen of about 28.75" with a horizontal of 20.5". Conveniently enough, this equates very closely to the desired 3:4 ratio, but on a rather large scale. Possibly too large for the aspect ratio to be important, as one would most likely not want to use the entire available space.
Given a display with enough resolution, both horizontal and vertical games could be represented well, so long as the game images can be placed within the screen real estate as desired. In another thread, I suggested that the way to get "perfect" 1:1 arcade representations would be to do something like this, using only a portion of the display, so this whole setup might be viable.
The only real downsides I can see are the cost of a 42" display and the "look" of the cabinet. For what a 42" display costs currently, one could buy a 27" multi-sync for true arcade resolutions, and a separate 23"-26" LCD for a marquee display. The previously mentioned "marquee on the same plane as the game screen" may also be a concern. Then again, it could also spawn a whole new look for arcade machine. Someone would have to give it a go and see what came of it

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RandyT